When you update to iOS 10.3, your iOS device will update its file system to Apple File System (APFS). This conversion preserves existing data on your device. However, as with any software update, it is recommended that you create a backup of your device before updating.

Quote As best I’ve been able to ascertain, these builds were available to download by anyone, but they were obscured by long, unguessable URLs. Someone within Apple leaked the list of URLs to 9to5Mac and MacRumors. I’m nearly certain this wasn’t a mistake, but rather a deliberate malicious act by a rogue Apple employee. Whoever did this is the least popular person in Cupertino. More surprises were spoiled by this leak than any leak in Apple history.